The present invention pertains to the type of well drilling bit commonly referred to as a "roller cone" bit or "rock" bit. Such a bit typically includes a main bit body which is attached to and rotates with the drill string. The bit body includes a centralized portion, which has the joint for attachment to the drill string at one end, and also includes a plurality of radially and circumferentially spaced legs depending longitudinally from the other end of the centralized portion. A rolling cutter body or "cone" is rotatably mounted on the free end of each of these legs. Thus, as the bit body is rotated by the drill string, these cones are caused to roll along the bottom of the hole being drilled, and teeth on the cones disintegrate the earth formation.
Rotary bearings are provided between the cones and their respective legs, and these bearings must be lubricated. An annular seal is provided at the free or open end of the bearing interface between the cone and the bit leg in order to keep the lubricant in the bearing and exclude well fluids, and the abrasives they carry, from the bearing. It is very important that the integrity of these bearing seals be maintained. If it is not, the bearing can be ruined. This not only makes repair difficult and expensive, if not impossible, but drilling must be temporarily stopped, and the drill string tripped to replace the bit, an extremely expensive operation.
Accordingly, such bits are typically provided with a respective lubricant reservoir recess for each cone, and in this recess is disposed a mechanism which contains a supply of lubricant, which can be urged toward the bearing through an interconnecting lubricant passage, to keep the bearing supplied and replace any lubricant which is lost; the mechanism also includes a flexible, preferably elastomeric, pressure compensator such as an elastomeric diaphragm, one side of which is exposed to the lubricant pressure, and the other side of which is exposed to the pressure external to the bit in the borehole. The compensator can react to increases in external pressure, so as to ensure that the lubricant pressure is equal thereto, and prevent leakage of drilling fluid into the bearing. Conversely, it can react to decreases in well pressure to reduce the pressure it is exerting on the lubricant, to prevent diaphragm damage, lubricant waste, and/or displacement of or damage to the bearing seal.
There are two basic types of such mechanisms known in the art, distinguished by whether the vent port for exposing the diaphragm to the external borehole pressure opens upwardly through the outer part of the bit body, or downwardly, through the shroud of the bit body.
Examples of so called "top vented" pressure compensators are shown in U.S. Pat. Nos. 3,942,596 and 4,942,930 as well as in the commercial literature of Reed, Walker McDonald, and Rock Bit International, filed herewith. An advantage of the top vented lubricator/compensator mechanism is that it is relatively simple in terms of the number and nature of parts, especially seals, required. In the hostile downhole environment, it is generally the case that, the simpler a mechanism can be, and more particularly the fewer its parts, the less vulnerability to damage, deterioration by well fluids and/or abrasives therein, damage from temperature and pressure conditions, etc.
However, top vented compensators generally suffer from some disadvantages. Specifically, because the vent port which exposes the compensator to the well fluid opens upwardly, this port will not drain or empty naturally, and will typically have to be cleaned out every time the bit is removed from the hole. What is worse, the port is near the outer extremity of the bit. Thus, as the bit is being pulled from the hole, material from the borehole wall may be scraped off:, falling onto, or even being driven into, the port and thence into the top of the mechanism and/or the recess in which it is housed. This can prevent prompt reuse of the bit, because, at best, extensive cleaning and repair may be required.
Examples of bottom vented mechanisms are shown in U.S. Pat. No. 4,865,136, U.S. Pat. No. 4,055,225, U.S. Pat. No. 4,727,942, and the commercial literature of Baker Hughes, filed herewith. The respective advantages and disadvantages of these bottom vented mechanisms are generally converse to those of the top vented mechanisms described above. More specifically, because the vent port opens downwardly, drilling fluid and cuttings can drain therefrom naturally, through force of gravity. Also, because these vent ports open through the shroud of the bit body, and are therefore isolated and protected from the wall of the borehole, there is no significant problem with scraping and cramming of formation into the vent ports and adjacent parts as the bit is pulled from the well. However, these mechanisms are generally more complex, not only utilizing at least two seals, but typically utilizing a greater number of parts in general.
Also, the elastomeric compensators of these mechanisms are typically perforated, rather than solid; in other words, they have integral pressure relief valves which can vent lubricant to the exterior of the bit if the pressure of the lubricant becomes too high and, for example, threatens to rupture the diaphragm and/or displace or damage the bearing seal of the associated cutter. This solution to the excess pressure problem can not only deplete the supply of lubricant available for feeding into the bearing, but because of the perforated nature of the compensator, can involve some danger of leakage of well fluid into the compensator, even though the integral valve thereof is designed to be of a one-way type.